1. Field of the Invention
Applicant's invention relates to a polymer manufacturing process that utilizes polyethylene terephthalate.
2. Background Information
Esterification is the condensation of organic carboxylic acids and alcohols to yield ester linkages. Polyesters are made when multifunctional carboxylic acids are reacted with multifunctional alcohols to yield polymers containing repeating ester units. Polyesters have become important polymer components used in a variety of industries.
The esterification reaction required to manufacture polyester polymers takes a great deal of time compared to other polymerization reactions. For example a typical aromatic polyester of moderate molecular weight can require between 12 and 24 hours to finish whereas an aromatic styrene polymer prepared by free radical polymerization can take as little as one hour to complete. There are several reasons for this increased duration. One reason is that the temperatures required for esterification are much higher on the order of 200 to 250 degrees Centigrade than those of other reactions such as free radical polymerization which require temperatures of only 70 to 100 degrees Centigrade. Another reason more time is needed is when higher molecular weight polyesters (such as those greater than 10,000 average number mw) are the goal. With higher molecular weight polyesters when the equivalents of hydroxyl and acid components are closer to being equal, the reaction becomes longer. While initially the reaction proceeds rapidly at first, once reaction temperatures have been reached the reaction starts slowing down as the free acids and hydroxyl groups become less concentrated in the mix. As the reaction slows additional steps and techniques are used to continue the reaction and create larger and larger molecular weights. With very large molecular weights (those greater than 30,000 mw) most often the material is transferred from the original vessel to one where more surface area, heat and/or agitation can be applied. Even larger molecular weights can be obtained by using additional processing steps such as solid state reactions or transesterification.
The following is an excerpt from Organic Chemistry by Morrison and Boyd, pages 679–680, second edition: “In the esterification of an acid, an alcohol acts as a nucleophilic reagent; in hydrolysis of an ester, an alcohol is displaced by a nucleophilic reagent. Knowing this, we are not surprised to find that one alcohol is capable of displacing another alcohol from an ester. This alcoholysis (cleavage by an alcohol) of an ester in called transesterification.” “Transesterification is an equilibrium reaction. To shift the equilibrium to the right, it is necessary to use a large excess of the alcohol whose ester we wish to make, or else to remove one of the products from the reaction mixture. The second approach is the better one when feasible, since in this way the reaction can be driven to completion.”
When making condensation polyesters, transesterification can be used as an additional processing step to achieve higher molecular weights with already condensed polymers or monomers. At higher temperatures the elimination of an alcohol and subsequent removal of it by vacuum will greatly increase molecular weight. At some point the end group can be liberated and removed by heat and/or vacuum thus building molecular weight. To extend the molecular weight of preformed polymers, transesterification sometimes follows an initial esterification step where the monomer mix, usually containing excess glycols, is first reacted to a point where most of the free carboxylic groups are used up. Or transesterification can be used alone to create polymers where the carboxylic groups have been pre-formed into esters with an easily volatilized alcohol, most commonly methanol. Thus both esterification and transesterification can be used separately or together in the process of making polyesters.
Over the years, many processes have been developed for manufacturing polyesters. In the 1940's it was discovered that polyester polymers could be made having very desirable properties such as clarity and high impact strength through the condensation of aromatic dicarboxylic acids with glycols using high temperatures and long reaction times to achieve higher molecular weights. By far the most important synthetic polyester today is polyethylene terephthalate (PET). This polymer is one where the multifunctional carboxylic acid is a terephthalate or terephthalic acid and the multifunctional alcohol is ethylene glycol. PET is a crystalline polymer that can be used for a variety of items such as film textile, fiber, beverage bottles, and other types of containers.
One method of making PET is to start with dimethyl terephthalate and transesterify with ethylene glycol liberating methanol. As methanol is removed from the process the molecular weight is driven up. Several transesterification catalysts have been used for this method. Due to the environmental problems associated with methanol, it has become more common to use terephthalic acid and ethylene glycol with a suitable esterification catalyst. Again there are a number of such catalysts used.
Esterification of terephthalic acid requires high temperatures, in excess of 200 degrees Centigrade, and long reaction times, sometimes longer than 24 hours. Thus it becomes a very energy intensive polymer to make. When very high molecular weights are needed, 50,000 or greater (which is considered low compared to other polymers), solid state reactors are used to vacuum as much glycol off as possible thus extending the chain length through transesterification and the removal of glycol. Additional heat and time are needed.
The PET manufacturing segment of the polymer industry has become so large that the cost of the raw materials of the PET polymer is low in comparison to other similar performance polymers. Large PET processing lines dedicated entirely to the manufacture of the PET polymers produce PET polymer on a continuous basis. Due to this production there has been a great deal of controversy over the large amounts of PET that are being recovered from post consumer waste streams. Due to this abundance of post consumer waste, there have been a large number of patents issued that concern the utilization of this PET waste. As we progress in the manufacturing techniques for other monomers and the need for higher performance materials become greater we will naturally expect to see the utilization of other condensation polymers to the point where they become prevalent in the waste streams. This has already started to happen with PET containing other barrier materials and with PEN or polyethylene napthalate.
As the waste stream from recycling started producing waste PET in abundance there were several patents written to utilize this potential raw material source. These patents became a technology in their own right. The first approaches to using PET were geared toward breaking down the ester linkages through hydrolysis with water or glycolysis. Glycolysis is a specific form of transesterification where excess glycol is used to degrade the molecular weight. In this way the individual components of the PET can be regenerated. In U.S. Pat. No. 4,078,143 issued to Malik, et al. entitled “Process for depolymerizing waste ethylene terephthalate polyester”, a process is described where PET is broken down by glycolysis to bis-(2-hydroxy ethyl) terephthalate, a monomer that can be utilized to reform the PET. In U.S. Pat. No. 4,163,860, issued to Delatte, et al. entitled “Process for obtaining dimethyl terephthalate from polyester scrap” methanol is used to transesterify scrap PET back to dimethyl terephthalate that is purified for use in the PET manufacturing process. In U.S. Pat. No. 4,355,175, issued to Pasztaszeri entitled “Method for recovery to terephthalic acid from polyester scrap”, a method of hydrolyzing the PET and purifying and recovering the terephthalic acid is described. In U.S. Pat. No. 4,578,502 issued to Cudmore entitled “Polyethylene terephthalate saponification process”, a process is described wherein PET is broken down into its monomeric constituents through saponification with alkali. In U.S. Pat. No. 4,929,749 issued to Gupta, et al. entitled “Production of terephthalate esters by degradative transesterification of scrap or virgin terephthalate polyesters”, higher boiling alcohols are used to transesterify the PET into lower molecular weight materials for use as raw materials for the manufacture of other polymers. In U.S. Pat. No. 5,101,064 issued to Dupont, et al. entitled “Production of terephthalate esters by degradative transesterification of scrap or virgin terephthalate polyesters”, a process is described where groups having 6–20 carbons are used to degrade the PET, distill off the glycol byproduct, and recover the diester.
In U.S. Pat. No. 5,266,601 issued to Kyber, et al. entitled “Process for preparing polybutylene terephthalate from PET scrap” a method of using PET by glycolysis and ester exchange with 1,4 butanediol and subsequent polycondensation is described. In U.S. Pat. No. 5,319,128 issued to Dupont, et al. entitled “Production of terephthalate esters by degradative transesterification of scrap or virgin terephthalate polyesters” a method of tranesterifying PET using higher molecular weight alcohols with 6 to 20 carbons and then recovering the diesters of terephthalate is described. In U.S. Pat. No. 6,031,128 and 6,075,163 issued to Roh, et al. entitled “Process for manufacturing terephthalic acid”, a process is described for manufacturing terephthalic acid from waste PET whereby PET is hydrolyzed to disodium terephthalate and then acid neutralized to recover the free terephthalic acid. In U.S. Pat. No. 6,472,557 issued to Pell, Jr. et al. entitled “Process for recycling polyesters”, a process for depolymerizing PET to dimethylterephthalate and then hydrolyzing it to terephthalic acid for reuse is described. Although all of these processes work, they are all very energy intensive ways of recycling the PET and do not utilize the time and energy that has already gone into making the PET polyester. More often these processes end up costing as much or even more than the cost of the monomers they are trying to reclaim. This is in large part due to the low cost of ii the beginning PET feed stocks and the refined methods for converting to the starting monomers. Also the additional energy required to reclaim the monomers from recycled PET adds substantially to the cost.
In the techniques used below, it is not necessary to take the PET polymer all the way to its monomeric constituents and thus at least part of the time and energy of conversion of the terephthalic acid and ethylene glycol is conserved. However in all cases the transesterification conversion is done to break down the PET linkages and lower the molecular weight to much lower oligomeric forms prior to subsequent reactions.
There are also a number of methods for the utilization of PET as a raw material for the manufacture of other polymers where terephthalic acid and/or ethylene glycol can be integrated as one of the components. One such area is in the use of PET to make polyols that in turn are used for making urethane foams. In U.S. Pat. No. 4,439,549 issued to Brennan entitled “Novel aromatic polyester polyol mixtures made from polyethylene terephthalate residues and alkylene oxides” a method of reacting PET with glycol to degrade to an oligomeric polyol and then subsequent reaction of the polyol with an isocyanate moiety to produce rigid foam is described. In U.S. Pat. No. 4,469,824 issued to Gigsby, Jr., et al. entitled “Liquid terephthalic ester polyols and polyisocyanate foams therefrom”, PET is digested with diethylene glycol and other glycols with some of the ethylene glycol and then removed to form a polyol that reacts with an isocyanate to form a polyisocyanate foam. In U.S. Pat. No. 4,485,196 issued to Speranza in entitled “Liquid phase polyols which are alkylene oxide adducts of terephthalic esters” a technique of making polyols for further processing into urethane foams is described. The polyol is further reacted by ethoxylation or propoxylation to liquefy and inhibit crystallinity. It is then useful for further conversion into polyurethanes. In U.S. Pat. No. 5,948,828 issued to Reck entitled “Technology development and consultancy” reclaimed PET is digested with diethylene glycol, insolubles are removed, and ethylene glycol and free diethylene glycol are removed to achieve a final hydroxyl value for a polyol. In U.S. Pat. No. 6,573,304 issued to Durant, et al. in June of 2003 entitled “Method for obtaining polyols and polyol thus obtained” a process for transesterification with glycols and subsequent removal of free glycols stopping at a narrow molecular weight is described. These methods utilize excess glycol and transesterification to shift the equilibrium back to lower molecular weight entities that can be further processed.
Some techniques developed utilizing PET have at least partially preserved some of the ester moieties and therefore some of the time and energy already used in making the PET. In U.S. Pat. No. 4,977,191 issued to Salsman entitled “Water-soluble or water dispersible polyester sizing compositions”, a process is described where other polymers are made by first degrading the PET into oligomers containing the terephthalate moiety and second building back up the molecular weight using other aromatic or aliphatic acids. In U.S. Pat. No. 5,726,277 issued to Salsman entitled “Adhesive compositions from phthalate polymers and the preparation thereof” adhesive compositions are described that are made from PET that is digested or transesterified with glycols and oxyalkylated polyols, either ethoxylated or propoxylated. A similar type of reaction is used in U.S. Pat. No. 5,958,601 issued to Salsman entitled “Water dispersible/redispersible hydrophobic polyesters resins and their application in coatings”. In this patent however an ester of a fatty acid and alcohol containing free hydroxyl groups is used in combination with glycols to degrade the PET polymer to lower molecular weight species before a molecular weight buildup is done with additional aromatic acids.
There are additional polymer applications where PET has been used as a raw material as well. In U.S. Pat. No. 5,820,982 issued to Salsman entitled “Sulfoaryl modified water-soluble or water-dispersible resins from polyethylene terephthalate or terephthalates” compositions are described which contain the terephthalate moieties along with sulfonated aromatic groups. Such resins are useful for adhesives, ink resins, dye leveling on polyester and nylon fibers, etc. The process for preparation of these compositions requires a PET glycolysis step followed by additional acids and a molecular weight buildup esterification step. The processing times can be 12 to 24 hours. In U.S. Pat. No. 6,133,329 issued to Shieh, et al. entitled “Thermoplastic polyester resin composition” a composition is described where PET is first digested with a glycol mixture for 3 hours at high temperatures and then reacted with a natural oil for making it compatible with hydrocarbon and hydrofluorocarbon blowing agents. In U.S. Pat. No. 6,512,046 issued to Ueno, et al. entitled “Polymerizable unsaturated polyester resin composition” several compositions are described where PET is first depolymerized to achieve a polyester skeleton, then built back up with a dibasic acid, and further reacted with an unsaturated monomer. In U.S. Pat. No. 6,534,624 issued to Ito, et al. entitled “Process for producing alkyd resins” a process is described where polyester is depolymerized and then esterified in a mixture of alcohols, glycols, fatty acids, etc. It is noted in this patent that the use of terephthalic acid has not been in practice in the past with alkyd technology because this component is more costly than phthalic or phthalic anhydride. Again all of these patents, some very recent, describe first a depolymerization step and then an esterification step to build back up molecular weight to make polymers suitable for other areas of use.
Other techniques deal with the use of reclaimed PET by cleaning up the PET from other wastes and using it as a co-blend prior to or in an extruder with virgin PET or other polymers that can be coextruded with the PET. Once reheated PET loses intrinsic viscosity (I.V.). Intrinsic viscosity has become a much easier method of comparing molecular weights of PET than other more time consuming methods. Once processed, the intrinsic viscosity drops and its use as a feedstock for the original article made becomes limited. In U.S. Pat. No. 5,225,130 issued to Deiringer entitled “Process for reclaiming thermally strained polyester scrap material” mixed streams of recycled PET are cleaned and post condensed with virgin PET. In U.S. Pat. No. 5,503,790 issued to Clements entitled “Method of producing disposable articles utilizing regrind polyethylene terephthalate” recycled PET is used to create articles that are less demanding of higher intrinsic viscosity. In U.S. Pat. No. 5,554,657 issued to Brownscombe, et al. entitled “Process for recycling mixed polymer containing polyethylene terephthalate” a process for recovering PET that involves dissolving the PET from a recycled stream, removing the solvents, and rinsing the PET is described. In U.S. Pat. No. 6,399,695 issued to Moriwaki, et al. entitled “Thermoplastic polyester resin composition” PET is melted with a polyolefin or glycidyl methacrylate to produce a composite material. In U.S. Pat. No. 6,583,217 issued to Li, et al. entitled “Composite material composed of fly ash and waste polyethylene terephthalate” the PET is mixed with the entitled materials and extruded. In the above references no reaction of the PET takes place even though there are subsequent reprocessing steps. There are many other references where recycled PET is cleaned and used as part of the mixture back into articles such bottles, film, etc. Limitations due to the lower intrinsic viscosity of the recycled PET reduce the amount used in critical applications to 5% or less.
There are also current practices where PET is modified by transesterifying with polyethers. These can be glycols or alcohols that have been ethoxylated or propoxylated. These polymers contain the block segments of PET with block segments of the polyethers and thus usually exhibit properties of both. In U.S. Pat. No. 4,785,060 issued to Nagler entitled “Soil release promoting PET-POET copolymer, method of producing same and use thereof in detergent composition having soil release promoting property” PET and a polyoxyethylene polymer are reacted together in a reactor such that an equilibrium is reached. This reaction is based on transesterification of the hydroxyl end groups of the polyether with the ester linkages contained in the PET. In U.S. Pat. No. 6,454,982 issued to Branum entitled “Method of preparing polyethylene modified polyester filaments” a method is described wherein polyethylene glycol is reacted into PET under transesterification conditions and solid stated to a higher intrinsic viscosity.
In the referenced prior art, glycols, polyethers, or simple glycol monoesters are used to degrade or lower the molecular weight of the PET in order to get to monomeric or oligomeric forms of terephthalic acid that can be further utilized as a polyol source for urethanes, to use as adhesive components with glycidyl ethers to form epoxies, or as coatings and/or adhesives.
Another polymer of commerce is polyethylene napthalate PEN. Within the last few years there has been much activity in the use of PET with PEN polymers. This is due in part to better properties such as clarity, strength, and increased crystallinity that translates to better barrier properties obtained with PEN. However, PEN is much more expensive than PET. Therefore, several processes for making copolymers of the two have been developed. In U.S. Pat. No. 5,902,539 issued to Schmidt, et al. entitled “Process for making PEN/PET blends and transparent articles therefrom” a process is described where ethylene glycol is used to reduce the intrinsic viscosity and increase the range of use for PET and PEN copolymers.
The following is an excerpt from U.S. Pat. No. 6,414,063, issued to Bassam, et al. entitled “Nucleated pet/pen polyester compositions”.
“It is known that medium content PET/PEN compositions (compositions with PET:PEN ratios around 50:50) are amorphous in nature. The range of compositions which display this amorphous behaviour is generally accepted to be around PET:PEN=20:80 to PET:PEN=80:20, as described by two PEN manufacturers—Shell (see FIG. 1 of presentation to “BevPak” conference, Spring 1995, U.S.A) and Hoechst-Trevira (page 4 of Polyclear.RTM. N technical literature). The disadvantage of this behaviour is that the use temperature of compositions in the 80/20–20/80 region is substantially reduced. The use temperature is dependent on the glass transition temperature (Tg) in this region. In contrast, the use temperature of PET/PEN compositions with <20% PET or <20% PEN is dependent on the crystalline melt temperature (Tm). Tm is over 100.degree. C. higher than the Tg for PET/PEN compositions; hence resulting in the substantial reduction in use temperature observed in the 20/80–80/20 composition region. The same observations on the amorphous/crystalline nature of PET/PEN compositions were also made by Lu and Windle (see FIG. 2 in Polymer 36 (1995), pages 451–459) and Andresen and Zachmann (Colloid & Polymer Science 272 (1994), page 1352). Andresen and Zachmann also found that blends of PET and PEN formed a single phase within 2 minutes of melting. This is usually good evidence for rapid formation of a PET/PEN copolyester by transesterification. Thus the behaviour of PET/PEN blends and copolymers can be expected to be the same with regards to crystallisation during all melt processing operations. In the case of bottle manufacture using PET/PEN copolymers and blends, U.S. Pat. No. 5,628,957 (to Continental PET Technologies Inc.) states that mid-range PET/PEN compositions containing 20–80% PEN are substantially amorphous and describes the use of an additional strain-hardenable (ie. crystallisable) layer for these mid-range PET/PEN bottles.”
It is especially interesting to note from this patent that the blends formed a single phase within 2 minutes of melting. Presumably from this and information presented in the description one can surmise that ester compatibility increases the rate of transesterification. Also, it can be inferred that PET and PEN copolymer combinations have been made via melting and/or processing since combinations of the two polymers were started. Again transesterification of the two is the chemisty that makes this happen. In U.S. Pat. No. 6,586,558 issued to Schmidt, et al. entitled “Process for making PEN/PET blends and transparent articles therefrom” glycols are used to lower the intrinsic viscosity and allow more processable viscosities for blends of these two polymers. Again transesterification allows this to occur.
While there has been a lot of activity directed toward utilizing PET as a raw material to manufacture other polymers or as a composite material, PET is not being utilized in these polymers as a raw material. The problems that exist with these prior techniques include raw material contamination, difficulty of reaction, and incompatibility with one or more of the other reactive groups. For instance, in U.S. Pat. No. 5,250,333 issued to McNeely, et al. entitled “Modified polyethylene terephthalate” there is described compositions where other alkoxylated polyols and dicarboxylic acids are used in combination with terephthalic acid and ethylene glycol to produce a less crystalline form of PET. Indeed there are many applications that use terephthalate moieties but require less crystallinity than that of PET. For instance, there are many film applications that require less crystallinity for more elastomeric properties. The polyols mentioned in the previous paragraphs are another example. In U.S. Pat. No. 6,428,900 issued to Wang entitled “Sulfonated copolyester based water dispersible hot melt adhesive” a polyester which contains water dispersible sulfonated branched copolyester polymers is described. These copolyester polymers use difunctional carboxylic acids like terephthalic acid in their makeup. Crystallinity would inhibit water redispersibility which is an important aspect of the disclosure. In U.S. Pat. No. 6,555,623 issued to Yang, et al. entitled “Preparation of unsaturated polyesters” a process is described where MPD (methyl propanediol) is used along with aromatic diacids such as terephthalic acid and maleic anhydride to produce unsaturated polyesters suitable for further reaction through the unsaturated group. Again polymer crystallinity is to be avoided.
There are a number of polymers that currently utilize phthalic anhydride as a preferred difunctional aromatic acid. One reason for this is that for practical considerations one of the acid groups has already been reacted and is an anhydride. This lowers the weight percent needed in the subsequent polymers being made. In addition phthalic anhydride esterifies at lower temperatures than terephthalic acid. Using terephthalic acid as an alternate would not be as economical to begin with. But terephthalic acid could be used if the right process to use recycled PET were available that would eliminate this economical difference.
There are a number of polymers containing ester linkages and the number and scope of polymers that utilize or could utilize the raw materials that make up PET or other condensation polymers of commerce are too numerous to list within the scope of this write. The following broad based articles of commerce all use or have used terephthalic acid (or aromatic acids like phthalic acid or anhydride) and/or ethylene glycol in their monomer makeup:
(1) Adhesives: either hot melt, water borne, or reactive;
(2) Ink resins: both as the binding agent and the carrier vehicle;
(3) Unsaturated resins: alone or in combinations with reactive diluents such as acrylics or styrene for composite mixtures with fiberglass, carbon fiber, etc.;
(4) Alkyd resins: both long and short alkyds for coatings and paint applications;
(5) Urethanes: As the polyol portion together with isocyanates to form adhesives, structural resins, or foams;
(6) Films: Less crystalline films for shrink wrap, laminating, etc.; and
(7) Polyols for powder coatings or fusable coatings.
As seen in the prior art, PET (either virgin or recycled) is recognized as a material that can be used to make more PET, PET composites, or other polymers that contain terephthalate groups. The processes that have been used to accomplish this contain within their steps glycolysis (or hydrolysis) of the ester linkages to create the beginning monomers such as terephthalic acid, or a much lower molecular weight terephthalate oligomer that can be reacted to generate more PET or other polymers through esterification. In no circumstance has there been activity that indicates advantage taken of the high molecular weight of PET (polyester polymers) being used to build higher molecular weight, on the order of 10,000 to 20,000, through transesterification with a lower molecular weight polyester.
The present process however does. The process of the present invention is a two-step process that can be used to take full advantage of the high molecular weight of the precondensed polymer, like PET, itself to produce a high molecular weight polymer. The first step, which involves no polymer of commerce, takes all the other monomers that are to be contained within the finished polymer, and reacts them to form a modifying polymer containing terminal hydroxyl or carboxyl groups. In the second step a commercially available condensation polymer is transesterified with the modifying polymer using heat and agitation to form the finished polymer. At suitable temperatures as will be herein discussed, the second step occurs very rapidly (on the order of approximately one-half of the current process) and can be performed in any vessel such as an extruder set up for reactive extrusion that can be heated to suitable transesterification temperatures, usually 230 to 270 degrees Centigrade for PET. The advantages of this process are to reduce the manufacturing time to a time on the order of approximately one-half of the current process and to produce a higher molecular weight polymer, on the order of greater than 192 g/mol, which relates to the decreasing time in a decreasing log/log curve of molecular weight versus time for direct esterification.